Many matrix-type printers, including inkjet printers, operate by repeatedly sweeping a printhead transversely over a print medium to print a number of dot rows. Each such sweep is referred to as a swath. The print medium is advanced longitudinally between each swath, so that the entire surface of the print medium is eventually covered.
As printing technologies improve and the pitch of the dot rows decreases, the accuracy with which the print medium advances becomes more critical. Inaccuracies in print medium positioning result in artifacts or bands on the printed page. For example, inaccuracies might cause two swaths to partially overlap, creating a noticeable and undesirable band of dot rows that has been printed twice.
Current media advance systems often use closed-loop servos to improve accuracy. However, increasing requirements for accuracy often limit the speed at which such servo systems can operate. Typically, significant inaccuracies and control system instabilities can be traced to drive train backlash. Accordingly, an effort is made to avoid overshoot when positioning the print medium, thereby avoiding backlash effects. To accomplish this, two positioning steps are often used. In a first step, the system slews at a relatively fast speed to an initial target position that is well short of the final desired position. The initial target position is selected so that the positioning step always stops short of the ultimate desired position, accounting for the worst case of positioning overshoot. In a second step, the servo system "creeps" forward very slowly to the final target. The slow speed avoids overshooting during this second positioning step, resulting in very accurate positioning.
During the slew portion of the positioning procedure, there is a variable amount of positioning error. This error, as an example, might vary between plus and minus four dot rows. To avoid overshoot, the initial target position is selected to be four dot rows short of the final target, thereby ensuring that overshoot will not occur. Given, however, that both positive and negative positioning errors might occur, the actual position attained during the slew positioning step might be as much as eight dot rows short of the final target position. Thus, the distance from the initial target position to the final desired position can be quite large. As a result, the second positioning step often accounts for a significant portion of the total media advance time, and throughput is severely restricted by the "creeping" of the print medium toward its final position.
Some printers provide a way to improve speed by implementing a "draft mode," in which the second positioning step is simply omitted. However, the quality of printing is noticeably poorer in this mode.